The present invention relates to an ion-conducting polymer membrane for a fuel cell, the polymer being formed of a polymer-forming hydrocarbon material, and to a method for producing such a membrane.
Ion-conducting polymer electrolyte membranes are used in fuel cells as membrane between anode and cathode. In this connection, conventional membranes have a minimum thickness of, typically, over 50 μm for the purpose of sufficient mechanical stability. On the other hand, the ionic conductivity of the membrane is limited by the thickness of the membrane.
If the intention is to make membranes as thin as possible, then it has been proposed for the ion-conducting polymer to be deposited as a thin layer on a carrier which is used as electrode. Such a membrane/electrode unit is described, for example, in European Patent Application EP-A-0 718 903.
Moreover, World Patent Application WO-A-95/19222 describes microcomposite polymers which are produced by mixing a solution of a polymer-forming hydrocarbon material, for example the fluorinated hydrocarbon which is available under the trade name Nafion®, with a metal alkoxide such as tetramethoxysilane, and subjecting it to a sol-gel process. In this context, the proportions of polymer-forming hydrocarbon material and of metal alkoxide are selected such that, in relation to the manufactured microcomposite polymer product, the proportion of fluorinated hydrocarbon material by weight is at most 80% and, accordingly, the proportion of metal alkoxide by weight is at least about 20%.
Subsequent to a treatment at elevated temperature, these polymers feature an inner of a metal oxide skeleton which gives rise to marked increase in the active surface of the polymer. The temperatures used are, at any rate, partially so high that they cannot be used for the polymer electrolyte membranes used in fuel cells. These polymers are said to possess a marked porosity featuring pore diameters of up to 1000 nm.
Unexamined European Laid-Open Patent Application EP 0 530 688 A2 describes a method for producing a fluorinated polymer with sulfonic acid side groups on a substrate, in which the substrate is wetted with a solution or dispersion containing the fluorinated polymer material whereupon the solvent or dispersant is driven off and the so obtained product is heated above the glass transition temperature of the polymer to make the polymer insoluble. The temperatures used in the process lie preferably between 150° C. and 300° C.
Unexamined German Laid-Open Patent Application DE 43 12 126 A1 described a gas diffusion electrode, for example for fuel cells, which includes a gas-permeable porous diffusion barrier which contains an electrically conductive catalyst in a finely divided form and is coated with a semipermeable membrane on its inner side. The membrane layer is composed, for example, of polysulfone, polyvinylidene fluoride, polyamide, or polyhydantoin, has a thickness of between 100 μm and 700 μm and can additionally contain fillers, preferably in a higher proportion compared to the polymer content. To manufacture the membrane, the filler, for example, titanium dioxide, zinc oxide, monocrystalline cellulose, silicic acid or aerosils (Degussa Company), is dispersed into a polymer solution which is then degassed in a vacuum and, upon application onto a substrate, is coagulated in water and subsequently washed and dried. A membrane which is made in a similar manner from a polymer-forming, perfluoro sulfonic acid containing solution, for example Nafion®, with introduced silicon dioxide particles, for example in the form of Aerosil 200 (Degussa Company) is disclosed in Unexamined European Laid-Open Patent Application EP 0 926 754 A1, both an amorphous and a crystalline phase being formed in this membrane via a controlled thermal treatment at a temperature above the glass transition temperature.